1. Field of the Invention
This invention relates to the production of olefins from a lower aliphatic alcohol, its corresponding ether or mixtures thereof. More particularly, it relates to the catalytic conversion of a methanol feed to an olefinic product. This invention especially relates to improvements in the conversion of a methanol feed to an olefinic product in a tubular reactor system.
2. Description of the Prior Art
The petrochemical industry has undergone tremendous growth in the past few decades. The production of synthetic fibers, plastics and petrochemicals by this and allied industries has steadily grown, at least in part, because of the availability of increasing supplies of inexpensive petrochemical raw materials, such as ethylene, propylene and other olefins. The principal source of ethylene at the present time is from steam cracked petroleum naphtha. The manufacture of polyethylene and styrene monomer utilizes a significant portion of this ethylene feed.
The ever-increasing demand for olefinic feedstocks has periodically caused a shortage of these basic raw materials, either because of a limitation in petroleum feedstocks of suitable quality or a limitation in naphtha cracking capacity. An alternate source of ethylene from non-petroleum sources is one obvious means of keeping pace with the demand for ethylene and other olefins.
In recent years, the patent art has disclosed that methanol and/or dimethyl ether, which may be obtained from coal, natural gas or biomass, can be converted to more complex hydrocarbons, such as olefins and aromatics, by utilizing a novel group of zeolites, exemplified by ZSM-5 zeolites. Ethylene is one of the olefinic hydrocarbons which may be obtained in this catalytic conversion. The reaction is highly exothermic and the olefins initially formed have a tendency to undergo further reaction to produce aromatic hydrocarbons useful in the production of motor gasoline. A large body of this patent art is concerned with various aspects of the conversion of methanol and/or dimethyl ether to light olefins, particularly ethylene.
The production of olefins from aliphatic ethers by catalytic conversion with a HZSM-5 zeolite catalyst is disclosed in U.S. Pat. No. 3,894,106 to Chang et al.
U.S. Pat. No. 3,979,472 to Butter discloses the conversion of lower alcohols and their ethers with a composite of antimony oxide and a ZSM-5 zeolite to produce a mixture of ethylene, propylene and mononuclear aromatics. U.S. Pat. No. 4,025,572 to Lago discloses that ethylene selectivity can be improved by diluting ZSM-5 with an inert diluent, while a similar result is achieved, according to U.S. Pat. No. 4,025,575 to Chang et al, through use of subatmospheric partial pressure of the feed. Selectivity of ethylene is also improved by employing ZSM-5 zeolite in large crystal form of at least 1 micron, either alone (U.S. Pat. No. 4,025,571 to Lago) or in combination, with added metals (U.S. Pat. No. 4,148,835 to Chen et al). Better selectivity is also obtained by interdispersing amorphous silica within the interior of the crystalline structure of the zeolite catalyst (U.S. Pat. Nos. 4,060,568 and 4,100,219 to Rodewald).
Although the above-described conversions perform exceptionally well and are unusually effective at converting lower aliphatic alcohols to olefinic hydrocarbons, it has been found that these conversions are exothermic to varying degrees, depending on the particular reactant. For example, the amount of heat generated in the conversion of the lower alcohols to hydrocarbon product may be estimated to be in the ranges shown.
______________________________________ Heat Produced, BTU per lb of Alcohol Reactant Hydrocarbon Product ______________________________________ Methanol 1000-2000 Ethanol 200-620 Propanol 15-360 ______________________________________
While it is desirable that a reaction be exothermic, since this obviates the need for an external source of heat to drive the reaction, large heat generation loads can require substantial investment in complex reactors with extensive internal cooling means. It can be seen from the above Table that the conversion of methanol, and to a lesser degree of ethanol, could be considered excessively exothermic in this regard. Furthermore, because of the inherent character and efficiency of the above-described crystalline aluminosilicate zeolite catalysts, the reaction of methanol, and to a lesser degree of ethanol, tend to be self-accelerating, thereby creating excessively hot local regions, where the reaction tends to go to completion, in the catalyst bed. These highly exothermic reactions can result in high catalyst aging rates, and possibly cause thermal damage to the catalyst. Furthermore, such high temperatures could cause an undesirable product distribution to be obtained. Therefore, it is critical in the conversion of methanol to useful products to provide sufficient heat dissipating facilities so that temperatures encountered in any portion of the catalyst sequence are restricted within predetermined limits.
Additionally, it is generally good engineering practice to conduct reactant conversions at elevated pressures to more effectively utilize the reactor volume and attendant equipment. With a methanol charge, however, elevated pressures tend to produce increased quantities of 1,2,4,5-tetramethylbenzene (durene), an undesirable by-product, while lower pressures, e.g., less than 50 psig, favor the production of light olefins.
Various techniques have been employed in controlling the exothermic heat released in the catalytic conversion of methanol: U.S. Pat. Nos. 3,931,349 to Kuo (use of light hydrocarbon diluents as heat sink for conversion of methanol-to-gasoline boiling products), 4,052,479 to Chang et al (operating conditions selected to restrict feed conversion to 5-25% ) and 4,238,631 to Daviduk et al (riser reactor and dense fluid catalyst bed). U.S. Pat. No. 4,035,430 to Dwyer et al describes arranging the catalyst in a series of beds of increasing size with interstage quenching with methanol, dimethyl ether and/or light hydrocarbons for controlling exothermic heat. A tubular reactor is disclosed in U.S. Pat. No. 4,058,576 to Chang et al as a means of removing exothermic heat during the catalytic conversion of a lower alcohol to olefins. A two-stage conversion is employed, with an alcohol dehydration catalyst utilized in the first stage and a ZSM-5 zeolite catalyst in the second stage, which is a tubular reactor. In one embodiment, the ZSM-5 catalyst is located in the tubes of the reactor, with a heat transfer medium passed through the shell side of the reactor. As the reaction mixture passes through the catalyst, the exothermic heat of reaction released within the tubes is transferred to the heat exchange medium. No details are provided on the configuration of the tubular reactor, insofar as it may influence the thermal stability of the desired reaction. There is also no recognition in prior art of the criticality of maintaining proper temperature within the reactor and controlling the flow rate of the reactants to maintain the degree of conversion at such a level as to maximize the yield of ethylene.
U.S. patent application Ser. No. 577,456, which is incorporated herein by reference, discloses a method of partial conversion of methanol to hydrocarbon products rich in ethylene and other light olefins by employing a tubular heat-exchanger type reactor. However, the tubular reactor described therein may not guarantee stable reactor operation at partial conversion if catalysts exhibiting pronounced autocatalysis are employed.
U.S. patent application Ser. No. 632,739 discloses a method for improving the exothermic conversion of methanol to light olefins in a multistage fixed bed adiabatic reactor system by cofeeding small quantities of light olefins to methanol.
A primary concern in the tubular reactor design is the controllability of the operation. It is thus an object of this invention to provide a process for converting lower aliphatic alcohols, their corresponding ethers or mixtures of said alcohols and ethers to an olefinic product comprising primarily ethylene in a thermally stable manner.
It is another object of the present invention to provide a tubular reactor for converting methanol, dimethyl ether (DME) or a mixture of methanol and DME to an olefinic product in a thermally stable manner.
These and additional objects will become apparent to those skilled in the art from the following description of the invention.